The original shonen "big three" are often compared and contrasted in many insightful ways, from their respective combat systems and worldbuilding to asking which protagonist is the strongest of the three. Smaller details can also be compared, such as their best first villain or even the best first mentor.
Nearly every shonen hero needs a first mentor, such as Satoru Gojo coaching Yuji Itadori and Tanjiro spending two years training with Mr. Urokodaki in the mountains. Meanwhile, Ichigo Kurosaki, Monkey D. Luffy and Naruto Uzumaki each needed an initial mentor to guide them, inspire them and teach them how to fight. In the context of being an early-era mentor in particular, it soon becomes clear which big three hero had the most compelling mentor.
Iruka Umino Was Naruto's First Real Teacher & Father Figure
Naruto Uzumaki had several mentors and father figures throughout Naruto, most notably the toad sage Jiraiya, who passed on his dream of world peace to his apprentice. However, in the early days, Naruto's best mentor was his ninja academy teacher, the chunin Iruka Umino. While Iruka was hard on Naruto for being a troublemaker and slacker, Iruka secretly cared deeply for the boy and even empathized with him, having lost his own parents in Kurama's attack 12 years ago. Iruka did his best to educate and train Naruto, and he risked his life to protect Naruto from the villainous Mizuki in Naruto's very first episode. When Naruto defeated Mizuki with shadow clones, Iruka was inspired to give Naruto his own Leaf ninja headband and proudly declare that Naruto had graduated ninja academy, starting Naruto's career as a proper genin.
Rukia Kuchiki Taught Ichigo How to Hunt Hollows & Purify Souls
Bleach's protagonist, Ichigo Kurosaki, trained with many characters to get stronger, ranging from Kisuke Urahara and Yoruichi Shihoin to his own father, the retired Soul Reaper Isshin Kurosaki. First, however, Ichigo needed coaching from his first mentor and Soul Reaper friend, Rukia Kuchiki. The two met in Karakura Town one fateful night as the Hollow called Fishbone D attacked, and Ichigo borrowed Rukia's powers to save the day. Rukia, without her powers, became a reverse-isekai hero and started living a double life with Ichigo, going to school by day and hunting Hollows in their spare time together. Rukia taught Ichigo all about spirits, Hollows, swordplay and her native Soul Society, but eventually, her foster brother Byakuya and his Lieutenant Renji arrived to take Rukia back home -- in chains.
Red-Haired Shanks Inspired Luffy With His Hat & a Promise
One Piece's popular hero the red-haired Shanks did relatively little to actually train Luffy, and they still often spend their time far apart from one another. In the "romance dawn" story arc though, Shanks protected Luffy from certain doom in the ocean, even giving up an arm to do so. Most of all, Shanks personally demonstrated what a real pirate captain looks like, being a wise, tough and crafty fellow who doesn't start fights but always ends them. Finally, Shanks inspired Luffy to become a great pirate captain with his encouraging words and his straw hat, his personal treasure. Ever since then, that moment has stuck in Luffy's mind, and Luffy has treasured that hat as a seemingly ordinary accessory that links him to his first mentor and Shanks' faith in him.
Iruka, Rukia & Shanks: Who's the Best Starter Mentor?
In the context of each anime's early arcs and each protagonist's need for a starter mentor and coach, Bleach's Rukia Kuchiki stands above the rest. True, in the long run, red-haired Shanks is stronger and even cooler than her, but as a starter mentor, Rukia delivers in ways that neither Iruka nor Shanks ever did. This is because Rukia wasn't just the one who taught Ichigo how to use a shonen combat system. She also served as an ambassador for all Soul Reapers, which was important, and she was also highly compelling as a reverse-isekai hero, giving her more depth than either Iruka or Shanks. This novelty helped Rukia stand out not just in Bleach but also among all shonen sidekicks.
In addition, anime fans absolutely adore Rukia and her tsundere ways, as she has excellent chemistry with Ichigo as his hotheaded but well-meaning friend who can gruffly beat him into shape the way no one else can. Rukia is strong, friendly, wise, vulnerable, funny and tough all at once, making her one of shonen's best-rounded mentors of all. It's little wonder Bleach fans ship her so passionately with Ichigo, giving rise to IchiRuki, one of anime's most enduring pairings. Rukia is many things to Ichigo, not just a first mentor, and in the eyes of some anime fans, "star-crossed lover" is included in that list. Few anime mentors can claim that title, canonically or not.